Messages in this thread | | | Date | Mon, 18 May 2009 15:59:31 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH] tracing: add trace_event_read_lock() |
| |
On Mon, May 18, 2009 at 07:35:34PM +0800, Lai Jiangshan wrote: > > I found that there is nothing to protect event_hash in > ftrace_find_event().
Actually, rcu protects it, but not enough. We have neither synchronize_rcu() nor rcu_read_lock.
So we protect against concurrent hlist accesses. But the event can be removed when a module is unloaded, and that can happen between the time we get the event output callback and the time we actually use it.
> RCU can also protect ftrace_find_event(), but it will add > latency to kernel. So this fix converts 'trace_event_mutex' > to a read/write semaphore, and adds trace_event_read_lock() > to protect ftrace_find_event(). > > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> > --- > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index 665a915..4ef8052 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -1569,12 +1569,14 @@ static void *s_start(struct seq_file *m, loff_t *pos) > p = s_next(m, p, &l); > } > > + trace_event_read_lock(); > return p; > } > > static void s_stop(struct seq_file *m, void *p) > { > atomic_dec(&trace_record_cmdline_disabled); > + trace_event_read_unlock(); > } > > static void print_lat_help_header(struct seq_file *m) > @@ -1817,6 +1819,7 @@ static int trace_empty(struct trace_iterator *iter) > return 1; > } > > +/* Called with trace_event_read_lock() held. */ > static enum print_line_t print_trace_line(struct trace_iterator *iter) > { > enum print_line_t ret; > @@ -3008,6 +3011,7 @@ waitagain: > offsetof(struct trace_iterator, seq)); > iter->pos = -1; > > + trace_event_read_lock(); > while (find_next_entry_inc(iter) != NULL) { > enum print_line_t ret; > int len = iter->seq.len; > @@ -3024,6 +3028,7 @@ waitagain: > if (iter->seq.len >= cnt) > break; > } > + trace_event_read_unlock(); > /* Now copy what we have to the user */ > sret = trace_seq_to_user(&iter->seq, ubuf, cnt); > @@ -3146,6 +3151,8 @@ static ssize_t tracing_splice_read_pipe(struct file *filp, > goto out_err; > } > > + trace_event_read_lock(); > + > /* Fill as many pages as possible. */ > for (i = 0, rem = len; i < PIPE_BUFFERS && rem; i++) { > pages[i] = alloc_page(GFP_KERNEL); > @@ -3168,6 +3175,7 @@ static ssize_t tracing_splice_read_pipe(struct file *filp, > trace_seq_init(&iter->seq); > } > > + trace_event_read_unlock(); > mutex_unlock(&iter->mutex); > > spd.nr_pages = i; > diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c > index 489c0e8..7136420 100644 > --- a/kernel/trace/trace_output.c > +++ b/kernel/trace/trace_output.c > @@ -14,7 +14,7 @@ > /* must be a power of 2 */ > #define EVENT_HASHSIZE 128 > > -static DEFINE_MUTEX(trace_event_mutex); > +static DECLARE_RWSEM(trace_event_mutex); > static struct hlist_head event_hash[EVENT_HASHSIZE] __read_mostly; > > static int next_event_type = __TRACE_LAST_TYPE + 1; > @@ -466,6 +466,7 @@ static int task_state_char(unsigned long state) > * @type: the type of event to look for > * > * Returns an event of type @type otherwise NULL > + * Called with trace_event_read_lock() held. > */ > struct trace_event *ftrace_find_event(int type) > { > @@ -475,7 +476,7 @@ struct trace_event *ftrace_find_event(int type) > > key = type & (EVENT_HASHSIZE - 1); > > - hlist_for_each_entry_rcu(event, n, &event_hash[key], node) { > + hlist_for_each_entry(event, n, &event_hash[key], node) { > if (event->type == type) > return event; > } > @@ -513,6 +514,16 @@ static int trace_search_list(struct list_head **list) > return last + 1; > } > > +void trace_event_read_lock(void) > +{ > + down_read(&trace_event_mutex); > +} > + > +void trace_event_read_unlock(void) > +{ > + up_read(&trace_event_mutex); > +} > + > /** > * register_ftrace_event - register output for an event type > * @event: the event type to register > @@ -533,7 +544,7 @@ int register_ftrace_event(struct trace_event *event) > unsigned key; > int ret = 0; > > - mutex_lock(&trace_event_mutex); > + down_write(&trace_event_mutex); > > if (WARN_ON(!event)) > goto out; > @@ -581,11 +592,11 @@ int register_ftrace_event(struct trace_event *event) > > key = event->type & (EVENT_HASHSIZE - 1); > > - hlist_add_head_rcu(&event->node, &event_hash[key]); > + hlist_add_head(&event->node, &event_hash[key]); > > ret = event->type; > out: > - mutex_unlock(&trace_event_mutex); > + up_write(&trace_event_mutex); > > return ret; > } > @@ -597,10 +608,10 @@ EXPORT_SYMBOL_GPL(register_ftrace_event); > */ > int unregister_ftrace_event(struct trace_event *event) > { > - mutex_lock(&trace_event_mutex); > + down_write(&trace_event_mutex); > hlist_del(&event->node); > list_del(&event->list); > - mutex_unlock(&trace_event_mutex); > + up_write(&trace_event_mutex); > > return 0; > } > diff --git a/kernel/trace/trace_output.h b/kernel/trace/trace_output.h > index 6e220a8..ac240e7 100644 > --- a/kernel/trace/trace_output.h > +++ b/kernel/trace/trace_output.h > @@ -20,6 +20,8 @@ extern int seq_print_user_ip(struct trace_seq *s, struct mm_struct *mm, > extern int trace_print_context(struct trace_iterator *iter); > extern int trace_print_lat_context(struct trace_iterator *iter); > > +extern void trace_event_read_lock(void); > +extern void trace_event_read_unlock(void); > extern struct trace_event *ftrace_find_event(int type); > > extern enum print_line_t trace_nop_print(struct trace_iterator *iter,
It could be more fine grained. We could have a per event rwsem, and also place the protected read section only in trace_print_entry() which is the only racy window.
But I'm not sure it's that worthy since event removal is a rare thing.
So I guess this patch is fine.
| |